Sources? Greed, basically. And that existed when one man realized he wants what somebody else has.
Part of the problem is lack of respect of law, esp. in relatively lawless areas, such as Africa. Nigeria is so famous for scammers, it has a scam named after them.
When people are left to fend for themselves, they tend to evolve a set of ethics that centers on self-reliance. If you got taken by a scam, you have only yourself to blame. With the power of global commerce, the scammers have a much larger... playing field.